Transfer characteristics of organic thin film and single crystal FETs

The threshold voltage and the turn-on voltage of pentacene thin film transistors (TFT) can be shifted by covering the gate insulator with a self assembled monolayer (SAM) of organosilane molecules. In this article we present experimental evidence identifying the SAM-induced modifications of the surface potential as the main cause for the shifted characteristics. To this end, FETs have been produced both on thin films and on single crystals. In the flip-crystal method for fabricating FETs, the single crystals of rubrene were placed onto prefabricated structures comprising a gate electrode, gate insulator and source/drain contacts. Prior to attaching the crystals, the SiO2 gate insulator was treated with different organosilane molecules that form SAMs on the gate insulator. Depending on the molecule's dipole moment, shifts of the characteristics by up to 40 V in V-g are measured. This behavior can be explained in a simple energy level diagram where the surface potential of the gate insulator is changed by the built in electric dipole field of the self assembled monolayer. (C) 2004 Elsevier B.V. All rights reserved.